1. Field of the Invention
The present invention relates to a MOSFET device which operates at high frequencies, particularly to a MOSFET device on an SOI (silicon-on-insulator) structure substrate, and a method of manufacturing the MOSFET device.
2. Description of the Related Art
In a high-frequency prior art MOSFET, operation at high frequencies can be improved by connecting a P.sup.+ diffused region 19 to a P.sup.+ semiconductor substrate (sub-layer) 1, just below a source electrode as shown in FIG. 8, thereby decreasing source inductance and ON-state resistance. To improve the high-frequency characteristics further, it is desired that the high-frequency MOSFET use an SOI substrate in which a thin film semiconductor layer having a thickness of several tens of nanometers is disposed on an insulating layer on a semiconductor substrate 1. However, such an insulating layer on a silicon semiconductor substrate makes it impossible to form the P.sup.+ diffused region connecting the semiconductor substrate in the same way as in the prior art. In FIG. 8, numeral 10 denotes a gate oxide film, 11 denotes a gate electrode, 12 denotes a CVD inter-layer film, 14 denotes a P type channel layer, 15 denotes an N.sup.- type LDD layer, 16 denotes N.sup.+ type source and drain layers, and 18a and 18b denote a source electrode and a drain electrode, respectively.
In order to overcome the connection problem, it has been proposed in Japanese Patent Kokai Nos. 7-283414 and 9-45789 to form a conductive portion that penetrates the insulating layer and connects to the P.sup.+ type substrate 1, just below the source electrode.
However, manufacture of an integrated circuit based on silicon, having a high degree of integration and high performance, requires wiring for the conductive portion with strips several micrometers wide in the SOI-MOSFET. Etching of an Si trench several micrometers wide before forming wiring for the source and the drain is difficult.
An SOI substrate has a low thermal conductivity because an SiO.sub.2 layer is interposed as an insulating layer, so that it is not suitable for a high power output device, such as a high-frequency MOSFET, which generates heat.